According to prior arts, it has been known to obtain SOI including a handle substrate composed of a transparent and insulating substrate and called Silicon on Quartz (SOQ), Silicon on Glass (SOG) and Silicon on Sapphire (SOS). It has been also known an adhered wafer obtained by bonding a transparent wide-gap semiconductor, including GaN, ZnO, diamond, AlN or the like, to a donor substrate such as silicon. The SOQ, SOG, SOS, etc., are expected to be applied to projectors, high-frequency devices and the like due to insulation and transparency of the handle substrate. Further, a bonded wafer that has a composite structure of a thin film made of the wide-gap semiconductor and the handle substrate is also expected to be applied to high-performance lasers, power devices and the like.
Such a composite substrate for semiconductors is comprised of the handle substrate and the donor substrate. In general, each of the handle substrate and the donor substrate is made up of a monocrystalline material. In the related art, a main method for forming these substrates involves forming a silicon layer on a base substrate by epitaxial growth. On the other hand, in recent years, the method of directly bonding them has been developed to contribute to improving the performance of semiconductor devices (see Patent Documents 1, 2, and 3). That is, such handle substrate and donor substrate are bonded together directly or via a bonding layer or an adhesive layer.
However, since sapphire is expensive, a substrate made of a material other than sapphire is desired to be used as the handle substrate in order to reduce the cost. With the progress in the bonding techniques described above, there are proposed various handle substrates that are made of a material other than sapphire, such as quartz, a glass, or alumina (see Patent Documents 4, 5, 6, and 7).
This kind of semiconductor device is essentially required to reduce its height together with increasing functionality and reduction of the size of an apparatus onto which the semiconductor device is to be mounted. The semiconductor device is thinned by processing, such as grinding, polishing, and etching, of one main surface (opposed surface) of the composite substrate formed by bonding, opposite to the other main surface thereof (main surface of the circuit side) with the semiconductor device formed therein. However, if the composite substrate is thinned, the warping of the substrate is more likely to occur, making it difficult to handle the substrate. For this reason, a method for enabling the handling of the composite substrate is known, which involves bonding a separate supporting body to the main surface on the circuit side of the composite substrate (see Patent Document 8).